Messages in this thread | | | From | Denys Vlasenko <> | Subject | Re: Nonblocking call may block in a mutex? Nonblocking call after poll may fail? | Date | Fri, 31 Aug 2007 15:32:26 +0100 |
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On Friday 31 August 2007 13:13, anon... anon.al wrote: > Hi! > > This is a driver-related question on non-blocking writes and poll. > > Setup: > there is a single output-buffer (in kernel-space) of 24 bytes for > writes from all processes A, B, and C: each process is restricted to > use at most 8 bytes: 8*3 = 24 > (until that data is handled (interrupt-handler...)) > > Question: > If this output-buffer has "4-bytes space remaining for process A", > then a non-blocking write of process A could still encounter a locked > mutex, if process B is busy writing to the output-buffer. > > Should process A now block/sleep until that mutex is free and it can > access the output-buffer (and it's 4 bytes space)? > > What about a non-blocking (write-) poll of process A: if the poll call > succeeds (the output buffer has space remaining for process A), and > process A now performs a non-blocking write: what happens if A > encounters a blocked mutex, since process B is busy writing to the > output-buffer. > a) Should A block until the mutex is available?
If mutex cannot be locked by B indefinitely, yes. If it can be locked indefinitely, then obviosly no.
> b) Should A return -EAGAIN, even though the poll call succeeded?
Succeeding poll is no guarantee against getting EAGAIN. When poll succeeds, it means "you _maybe_ can write now".
> c) Should it be impossible for this to happen! i.e. -> should process > A already "have" the mutex in question, when the poll call succeeds > (thus preventing B from writing to the output buffer)
No. Kernel cannot know whether A will do the write at all.
> For c) What if process A "has" the mutex, but never does the > non-blocking write. Then no process can write, since the mutex is held > by process A...
Exactly. (c) would be kernel bug. -- vda - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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